The conversion of natural gas assets into more valuable products, including combustible liquid fuels, is desired to more effectively utilize these natural gas assets. The conversion of natural gas to more valuable products generally involves syngas generation. Syngas generation involves converting natural gas, which is mostly methane, to synthesis or syngas gas, which is a mixture of carbon monoxide and hydrogen. Syngas may be used as a feedstock for producing a wide range of products, including combustible liquid fuels, methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates.
There are two main approaches to convert remote natural gas assets into conventional transportation fuels and lubricants using syngas. Natural gas may be converted into syngas followed by a Fischer-Tropsch process, or natural gas may be converted into syngas followed by methanol synthesis, which is followed by a methanol to gas process (MTG) to convert methanol into highly aromatic gasoline. The syngas generation is the most costly step of these processes. A critical feature of these processes is producing syngas with a desired H2/CO ratio to optimize formation of the desired products and to avoid problems in the syngas formation step. Unless otherwise stated, syngas ratios (and percentage compositions) as described herein are in terms of molar ratios (and molar percentages).
Syngas can be generated from three major chemical reactions. The first involves steam reforming of methane. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 3.0. A second process for syngas generation involves dry reforming of methane or the reaction between carbon dioxide and methane. An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reactor. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0. A third process for synthesis gas generation involves partial oxidation of methane using oxygen. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0.
It is possible to produce syngas with a H2/CO ratio that is above the ratio ideally desired for the process in which the syngas is to be used, and then to remove excess hydrogen to adjust the ratio to the desired value. However, the H2 removal process employs expensive H2 separation systems that tend to foul and decline in performance with use.
The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into saleable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having low crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels. Furthermore, like the Fischer-Tropsch process, the MTG process also generates CO2.
It is known to use multiple Fischer-Tropsch reactors for various purposes. For instance, U.S. Pat. No. 6,169,120, to Beer of Syntroleum, discloses a two-stage Fischer-Tropsch process employing intermediate H2O removal. The process of Beer is intended to be used to process syngas produced from air that will contain appreciable amounts of nitrogen.
U.S. Pat. No. 4,279,830, to Haag et al. of Mobil, discloses a zeolite-containing catalyst used to maintain a syngas composition so that a H2:CO ratio is in a range of about 0.5 to about 1.0. Haag explains that this range is appropriate for Fe-based FT catalysts, but is not suitable for Co-based catalysts. A second FT catalyst contains Fe but does not contain a zeolite. This catalyst makes a minimal change in the H2:CO ratio. Haag also explains that operating within a selective range of process conditions provides an improved process for upgrading the total effluent from a Fischer-Tropsch operation.
U.S. Pat. No. 4,624,968, to Kim et al. of Exxon, discloses a two-stage Fischer-Tropsch process using specific catalysts intended for olefin synthesis and conversion.
WO/0063141, to Clark et al. of Reema International Corp., discloses a Fischer-Tropsch process for synthesizing hydrocarbons that includes multiple Fischer-Tropsch reactor stages arranged in series. The process of Clark provides very low CO conversion per Fischer-Tropsch reactor stage and employs intermediate removal of water between reactor stages. In one embodiment, the process of Clark uses an iron-based catalyst. In a preferred embodiment, CO2 is recycled from the last reactor in a series of Fischer-Tropsch reactors to the syngas generator.
EP 0679620 A2, to Long of Exxon, discloses a high conversion hydrocarbon synthesis achieved by reacting H2 and CO in a first stage(s) in the presence of a non-shifting catalyst. The process further includes separating liquid products and reacting the remaining gas streams in the presence of shifting catalysts.
Additionally, various technologies have been used for the conversion of CO2. For instance, “Can Carbon Dioxide be Reduced to High Molecular Weight in Fischer-Tropsch Products?,” In re Puskas, ACS 213 National Meeting (San Francisco Apr. 13–17, 1997) ACS Division of Fuel Chemistry Preprints, V. 42, N. 2, 680–86 (1997), discloses that CO2 can be converted into hydrocarbonaceous products of similar structure and composition as obtained with CO. Also, “Kinetics of CO2 hydrogenation on a K-promoted Fe Catalyst,” Thomas Riedel and Georg Schaub, Industrial and Engineering Chemistry Research, 40/5 1355–1363, discloses that CO2 can be converted into hydrocarbonaceous products of similar structure and composition as obtained with CO. Riedel and Schaub discloses that preferred operating temperatures are below 360° C. to prevent rapid carbon deposition on the catalyst.
Similarly, “Iron Catalyzed CO2 Hydrogenation to Liquid Hydrocarbons,” Fourth International Carbon Dioxide Utilization Conference (Kyoto, Japan Sep. 7–11, 1997) Studies in Surface Science and Catalysis, V114, 339–44 (1998), discloses that many catalysts useful in Fischer-Tropsch synthesis can also catalyze CO2 hydrogenation to hydrocarbons.
Although multiple Fischer-Tropsch reactors have been used, an integrated system employing multiple Fischer-Tropsch reactors wherein hydrocarbonaceous products from different stages are blended has not been utilized to reduce CO2 emissions.
As a result, there is an urgent need for a process that can reduce CO2 emissions from syngas conversion processes while still being able to generate desired hydrocarbonaceous products.